For well over 100 years aircraft of numerous types have been proposed—and some have been successfully demonstrated and commercially practiced. However many proposed aircraft designs have been limited to paper proposals and/or one or a few demonstration models rather than achieving commercial practicality. This is perhaps especially the case for VTOL craft designs which often encounter onerous flight control requirements—especially when considering the options for a safe descent to landing after some critical aircraft system or subsystem failure.
The need for an effective, reliable and safe flight control system (FCS) is especially critical for modern fly-by-wire (FBW) or future fly-by-light (as optical fiber and/or solid state optical data communication and control circuitry becomes more commonly used) aircraft where control of all critical flight control parameters can be effected by computerized control systems. Pilot controlled input sensors (possibly located remotely in the case of un-manned aircraft) generate electrical command control inputs to a computer control system which also receives feedback electrical inputs representing the actual current state of the aircraft (e.g., its position, attitude, speed, etc). Based on such electrical inputs, the computerized control system generates electrical output signals that are routed to electromechanical actuators or other suitable transducers to control critical aircraft control parameters (e.g., aircraft control surfaces and systems, etc.).
Some very high performance aircraft designs are inherently nearly unstable and thus perhaps even incapable of sustained direct human control without such a FBW FCS. In an attempt to provide enhanced reliability and safety, redundancies have often been built into the FBW FCS. However such prior systems have typically attempted to maintain 100% control power (CP) of the entire aircraft in the event of a failure in one of the redundant sub-systems by substituting a backup subsystem for the failed one. Typically the process of deciding that a subsystem failure has occurred and what responsive action to take is also computerized (e.g., by a supervising computer system which itself might have some built in redundancy). The required complexity of hardware and software components for such a FBW FCS has led to inevitable design and/or implementation flaws that can cause, and have caused, catastrophic unexpected results.
For example, in a typical FBW FCS, particularly for aircraft with reduced or “relaxed” stability (such as high performance military aircraft, helicopters, hovercraft and the like), there may be one relatively large redundancy system that may include, for example, a first nominal primary computer, second and third back-up computers, and a fourth computer that monitors the first three and “votes” or selects the best of the first three (based on performance data) as the primary and one of the remaining two as the secondary. Such systems are highly software-dependent and, on occasion, the computer makes the wrong decision. In addition, if an actuator in the system fails, it cannot be fixed but only replaced in the event one or more redundant actuators are incorporated into the system.
Recent advances in computers, sensors and other technologies have made possible reliance on unmanned air vehicles (UAVs) where no human pilot is present on board. In these vehicles, the pilot operated controlled input sensors that generate electrical command control inputs to a computer control system are not used. However such electrical command control inputs are still necessary for commanding the vehicle and are either commanded from the ground through a wireless communication link or by a computerized on-board system, sometimes termed mission-control-computer or mission-control-system, or both. The needs of UAVs in terms of reliability and redundancy are not materially different from those described earlier for manned systems and therefore the advantages of the FCS described herein are also applicable to UAVs.
In another FCS developed by Boeing for use with a fixed-wing aircraft (see U.S. Pat. No. 4,598,890) a multi-data bus or multi-channel redundancy FBW system features complete channel separation with no automatic switching of data or control information between the channels for a given aircraft control surface (e.g., the elevator). The system as described was supposed to briefly sustain a failure of any one segment of a multi-segmented control surface, e.g., the elevator control surfaces (on the aircraft horizontal stabilizer) which are physically split apart into four independently movable segments while it was being returned in a failsafe manner to a neutralized position. This prior system also used a visual feedback system to the pilot and contemplated a manual pilot input to de-activate and neutralize any malfunctioning one of the control segments.
Many different types of VTOL vehicles have been proposed where the weight of the vehicle in hover is carried directly by vertical air flow generated by rotating fans, propellers, turbojets, etc. with the axis of rotation generally perpendicular to the ground. One well known vehicle of this type is the conventional helicopter which includes large rotor blades mounted above the vehicle fuselage. Other types of VTOL vehicles have used propellers installed inside circular cavities, shrouds, ducts or other types of nacelle (generically referred to herein as a duct or ducts), where the propeller or rotor blades are not exposed, and where the flow of air is generated inside the duct. Most such ducts have uniform circular cross-sections with the exit area (usually at the bottom of the duct when the vehicle is hovering) being similar to that of the inlet area (usually at the top of the duct). Some ducts, however, have other cross-sections and some are slightly divergent, having an exit area that is larger than the inlet area, as this was found to increase efficiency and reduce the power required per unit of lift for a given inlet diameter. Some ducts have a wide inlet lip in order to augment the thrust obtained, especially in hover.
Other types of VTOL vehicles have used a multitude of smaller rotors or propellers, usually two or sometimes four, mounted at the tip of a wing or tandem wings, and having the ability to be oriented vertically for obtaining VTOL and then the ability to be tilted forward in a manner similar to fixed wing aircraft for providing thrust while the wing(s) provide(s) lift. These vehicles are often termed “tilt-rotor”. Some of these VTOL vehicles have used circular ducts surrounding the rotors or propellers to achieve the previously mentioned advantages possible with a duct, while retaining the tilting capability of the “tilt-rotor”. Such vehicles are often termed “tilt-duct”.
VTOL vehicles are usually more challenging than fixed wing aircraft in terms of stability and control—and hence the need for more effective, reliable and safe FCSs. One difficulty rises from the fact that, contrary to fixed wing aircraft which accelerate on the ground until enough airspeed is achieved to develop minimum relative air velocity on their flight surfaces for well controlled flight, VTOL vehicles may sometimes hover with little or zero movement relative to the ground and/or air masses of the surrounding ambient air. For these vehicles, the control system typically utilizes, for example, the pitch and/or rotational speed of the rotors or propellers or the flow of air that they produce to create control forces and moments around the vehicle's center of gravity (CG) so as to controllably direct the effective thrust centerline created by the rotor or propeller.
One method, which is very common in helicopters, is to mechanically change, by command from the pilot, the pitch of the rotating rotor blades both collectively and cyclically, and to modify the main thrust as well as moments and/or inclination of the propeller's thrust line that the propeller or rotor exerts on the vehicle. Some VTOL vehicles using ducted or other propellers that are mounted inside the vehicle also employ this method of control. Some designers choose to change only the angle of all the rotor blades using ducted or other propellers mounted inside the vehicle for this method of control. The angle of all the rotor blades may be changed collectively (termed collective control) to avoid the added complexity of changing the angle of each blade individually (termed cyclic control). On vehicles using multiple fans or rotor blades which are relatively far from the CG, different collective control settings can be used on each fan or rotor blade to produce the desired control moments.
The disadvantage of using collective controls, and especially cyclic controls, lies in their added complexity, weight and cost. Therefore, a simple thrust unit that is also able to generate moments and side forces, while still retaining a simple rotor not needing cyclic blade pitch angle changes, has an advantage over the more complex solution. The main problem is usually the creation of rotational moments of sufficient magnitude required for control.
One traditional way of creating moments on ducted fans is to mount a discrete number of directional force control vanes only at or slightly below the exit section of the duct. These vanes, which are immersed in the flow exiting the duct, can be deflected to create a side force. Since the vehicle's center of gravity is in most cases at a distance above these vanes, the side force on the vanes also creates a moment around the vehicle's CG.
However, one problem associated with vanes mounted only at the exit of the duct in the usual arrangement as described above, is that even if these are able to create some force or moment in the desired direction, they cannot do so without creating at the same time a significant further side force or moment that has an unwanted secondary effect on the vehicle. For such vanes mounted only below the vehicle's CG (which is the predominant case in practical VTOL vehicles), these side forces cause the vehicle to accelerate in directions which are usually counter-productive to the result desired (e.g., through the generation of undesired additional forces and/or moments by the same vanes), thereby limiting their usefulness on such vehicles.
The prior Chrysler VZ-6 VTOL flying car used vanes on the exit side of the duct, together with a small number of very large wings mounted outside and above the duct inlet area. However, in the VZ-6, the single wing and the discrete vanes were used solely for the purpose of creating a steady, constant forward propulsive force, and not for creating varying control moments as part of the stability and control system of the vehicle.
The prior Hornet unmanned vehicle developed by AD&D, also experimented with using either a single, movable large wing mounted outside and above the inlet, or, alternatively using a small number of vanes close to the inlet side. However these were fixed in angle and could not be moved in flight for control purposes.
Another case that is sometimes seen is that of vanes installed radially from the center of the duct outwards, for the purpose of creating yawing moments (around the propeller's axis).